1. Field of the Invention
This invention relates to a device for coating a continuous substrate and in one aspect to an apparatus and method for electrospraying a coating material onto a substrate.
2. Description of the Prior Art
A number of substrate coating methods are presently available. Mechanical applications such as roll coating, knife coating and the like are easy and inexpensive in themselves. However, because these methods give thick coatings of typically greater than 5 micrometers (um), there are solvent to be disposed of and this disposal requires large drying ovens and pollution control equipment, thus making the total process expensive and time consuming. These processes are even more awkward for applying very thin coatings, for example, less than 500 Angstroms (.ANG.). To apply such thin coatings by present coating techniques requires very dilute solutions and therefore very large amounts of solvent must be dried off. The uniformity and thickness of the dried final coating is difficult to control.
Physical vapor deposition techniques are useful for applying thin and very thin coatings on substrates. They require high vacuums with the attendant processing problems for a continuous process and are therefore capital intensive. They also can only coat materials that can be sputtered or vapor coated.
The present invention relates to an electrostatic spraying process but it is unlike conventional electrostatic processes which have been used for a number of years. Such processes for example, are used in the painting industry and textile industry where large amounts of material are applied to flat surfaces wherein application of such coatings use a droplet size in the 100 micrometer range with a large distribution of drop sizes. Uniform coatings thus start at about 200 micrometer thickness, which are thick film coating processes. Significant amounts of solvents are required and these solvents do not evaporate in travel from sprayer to substrate so the coating is a solvent wet coating which then requires drying. It is difficult to coat nonconductive substrates with these processes. The spray head design for these electrostatic coating processes usually are noncapillary and designed so that the charged material to be coated comes off a sharp edge or point and forms very large droplets. For example, Ransburg, U.S. Pat. No. 2,893,894 shows an apparatus for coating paints and the like from an electrostatic spray gun. Probst, U.S. Pat. No. 3,776,187 teaches electrostatic spraying of carpet backings from a knife edge type apparatus.
Liquid jet generators for ink jet printing are a controlled form of electrostatic spraying. In ink jet generators, streams of drops of liquid on the order of 75 to 125 micrometers in diameter are produced, charged and then guided in single file by electric fields along the drop stream path to the desired destination to form the printed character. Sweet, U.S. Pat. No. 3,596,275 describes such a generator wherein the series of drops are produced by spaced varicosities in the issuing jet by either mechanical or electrical means. These drops are charged and passed one by one through a pair of electrostatic deflecting electrodes thereby causing the writing to occur on a moving substrate beneath the generator.
Van Heyningen, U.S. Pat. No. 4,381,342 discloses a method for depositing photographic dyes on film substrates using three such ink jet generators as just described in tandem and causing each different material to be laid down in a controlled non-overlapping matrix.
The design of structures to generate small charged droplets are different from the aforementioned devices for painting and jet printing. Zelany, Physical Review, Vol. 3, p. 69 (1914) used a charged capillary to study the electrical charges on droplets. Darrah, U.S. Pat. No. 1,958,406, sprayed small charged droplets into ducts and vessels as reactants because he found such droplets to be "in good condition for rapid chemical action".
In an article in Journal of Colloid Science, Vol. 7, p. 616 Vonnegut & Neubauer (1952) there is a teaching of getting drops below 1 micrometer in diameter by using a charged fluid. Newab and Mason, Journal of Colloid Science, Vol. 13, p. 179, (1958) used a charged metal capillary to produce fine drops and collected them in a liquid. Krohn, U.S. Pat. No. 3,157,819, showed an apparatus for producing charged liquid particles for space vehicles. Pfeifer and Hendricks, AIAA Journal, Vol. 6, p. 496, (1968) studied Krohn's work and used a charged metal capillary and an extractor plate (ground return electrode) to expel fine droplets away from the capillary to obtain a fundamental understanding of the process. Marks, U.S. Pat. No. 3,503,704 describes such a generator to impart charged particles in a gas stream to control and remove pollutants. Mutoh, et al, Journal of Applied Physics, Vol. 50, p. 3174 (1979) described the disintegration of liquid jets induced by an electrostatic field. Fite, U.S. Pat. No. 4,209,696, describes a generator to create molecules and ions for further analysis and to produce droplets containing only one molecule or ion for use in a mass spectrometer and also describes the known literature and the concept of the electrospray method as practiced since Zeleny's studies. Mahoney, U.S. Pat. No. 4,264,641, claimed a method to produce molten metal powder thin films in a vacuum using electrohydrodynamic spraying. Coffee, U.S. Pat. No. 4,356,528 and U.S. Pat. No. 4,476,515 describes a process and apparatus for spraying pesticides on field crops and indicates the ideal drop size for this application is between 30 and 200 micrometers.
The prior art does not teach an electrostatic coater for applying a coatings 10 to 5000 .ANG.. thick at atmospheric pressure.
The prior art does not teach the use of a coater with a wide electrostatic spray head having a plurality of capillary needles.